Electrical superconductivity is a well-known phenomenon that arises in certain materials and makes their electrical resistivity practically zero.
This property of certain materials is particularly advantageous because it results in the ability to produce windings for generating magnetic fields that can handle high electrical currents, as long as certain critical current densities are not exceeded, without joule heating and with smaller conductive sections, and thus lower coil masses.
However, in order to obtain this behavior of the material, it is necessary to maintain it at temperatures below a critical temperature that depends on the conductive material used, which temperatures may be cryogenic and near absolute zero, at least for certain modes of superconductivity.
This constraint resulted in the development of machines with superconducting coils limited to static applications, such as for example coils used in particle accelerators or magnetic resonance imaging devices, and more recently for storing energy in magnetic form, for which the continuous cooling required may be achieved without any insurmountable difficulty by installations that are heavy and complex to implement.
The discovery of so-called “high-temperature” superconducting materials, for example magnesium diboride MgB2, whose superconductivity is obtained at temperatures on the order 30 Kelvin, or other alloys which can have superconductivity at temperatures as high 70 Kelvin, has made it possible to reduce the temperature constraints and simplify the cooling systems of machines using superconducting coils.
The European patent application published under the number EP 1777800 describes an example of an electromechanical machine using a superconducting coil.
In this example, the superconducting coil, located between the rotor and the stator, is enclosed in a cryostat placed inside the machine so that the coil is maintained at a temperature lower than the critical temperature of the material used. No explanation of how the cryostat is maintained at the desired temperature is given.
As is known, the maintenance of such a low temperature is obtained by means of a fluid, for example liquid nitrogen, helium, or liquid hydrogen depending on the critical temperature of the superconducting material used, maintained at the desired temperature by cooling systems.
In a first known method, a reservoir of low-temperature liquefied gas is used as a cold supply and a flow is drawn from this reservoir in order to continuously cool the machine's electrical conductors made of superconducting material before being discharged to the outside of the machine.
In that case it is necessary to provide a sufficient mass of liquefied gas at a cryogenic temperature, storage means for maintaining this gas at its liquefaction temperature, and regulating means for distributing the gas so as to maintain the superconducting elements of the machine at the desired temperature while limiting the consumption of gas to a minimum.
When the machine is onboard a vehicle, in addition to the cumbersome necessity of carrying a sufficient quantity of gas, it is necessary to provide distribution and regulating means whose operation at a cryogenic temperature is more complex than in the case of systems operating at ambient temperature.
In a second known method, a low-temperature fluid circulates in a closed circuit between the parts of the machine to be maintained at a cryogenic temperature and a cryogenic cold-generating device (known as a cryocooler).
Such cryogenic cold-generating devices are known, but they are still heavy and bulky and must be supplied with energy in order to produce the required cold.
In cases where such devices are used, a malfunction of the cooling system generally results in an immediate shutdown of the machine being cooled unless redundant cooling devices are provided.
The known methods are therefore disadvantageous and are unsatisfactory for onboard machines when, in particular, volume, mass, and reliability are essential criteria, such as for example in the case of applications onboard aircraft.